The main services and functions of the MAC sub-layer include:
- Mapping between logical channels and transport channels;
- Multiplexing/demultiplexing of MAC SDUs belonging to one or different logical channels into/from transport
blocks (TB) delivered to/from the physical layer on transport channels; - Scheduling information reporting;
- Error correction through HARQ (one HARQ entity per cell in case of CA);
- Priority handling between UEs by means of dynamic scheduling;
- Priority handling between logical channels of one UE by means of logical channel prioritisation;
- Padding.
Logical Channels:
A single MAC entity can support multiple numerologies, transmission timings and cells. Mapping restrictions in logical channel prioritisation control which numerology(ies), cell(s), and transmission timing(s) a logical channel can use.
Different kinds of data transfer services as offered by MAC. Each logical channel type is defined by what type of information is transferred. Logical channels are classified into two groups: Control Channels and Traffic Channels.
Control channels are used for the transfer of control plane information only:
- Broadcast Control Channel (BCCH): a downlink channel for broadcasting system control information.
- Paging Control Channel (PCCH): a downlink channel that transfers paging information, system information
change notifications and indications of ongoing PWS broadcasts. - Common Control Channel (CCCH): channel for transmitting control information between UEs and network.
This channel is used for UEs having no RRC connection with the network. - Dedicated Control Channel (DCCH): a point-to-point bi-directional channel that transmits dedicated control
information between a UE and the network. Used by UEs having an RRC connection.
Traffic channels are used for the transfer of user plane information only: - Dedicated Traffic Channel (DTCH): point-to-point channel, dedicated to one UE, for the transfer of user
information. A DTCH can exist in both uplink and downlink.
Mapping to Transport Channels:
In Downlink, the following connections between logical channels and transport channels exist:
- BCCH can be mapped to BCH;
- BCCH can be mapped to DL-SCH;
- PCCH can be mapped to PCH;
- CCCH can be mapped to DL-SCH;
- DCCH can be mapped to DL-SCH;
- DTCH can be mapped to DL-SCH.
In Uplink, the following connections between logical channels and transport channels exist:
- CCCH can be mapped to UL-SCH;
- DCCH can be mapped to UL- SCH;
- DTCH can be mapped to UL-SCH.
HARQ:
The HARQ functionality ensures delivery between peer entities at Layer 1. A single HARQ process supports one TB when the physical layer is not configured for downlink/uplink spatial multiplexing, and when the physical layer is configured for downlink/uplink spatial multiplexing, a single HARQ process supports one or multiple TBs.
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Reference :
3GPP TS 38.300 Rel 15 Ver 15.3.0